Alignment system

ABSTRACT

This alignment system utilizes intermediate photodetectors having central apertures and a terminal photodetector, each photodetector having four quadrants of active area, with their centers aligned on the axis of a laser beam, their output signals being utilized to indicate alignment, or the degree of misalignment, of their centers with respect to the axis of the laser beam. Alternatively their output signals are utilized to drive servo systems that automatically move the quipments on which they are mounted into alignment, Means for providing orthogonal alignment, and means to avoid air turbulence are also provided.

United States Patent 1 11 3,723,013 Stirland et al. 1 Mar. 27, 1973 54]ALIGNMENT SYSTEM 3,470,377 9/l969 Le Febre et all ..356/l52 [75]Inventors: Meade A. Stirland, Los Alamos, N.

John Kalinowski, 3,603,688 9 1971 Smith-Vaniz ..356/152 Ramon, Calif.

The United States of America as represented by the United States AtomicEnergy Commission Filed: Oct. 23, 1970 Appl. No.: 83,567

Assignee:

References Cited UNITED STATES PATENTS Primary ExaminerBenjamin A.Borchelt Assistant Examiner-S. C. Buczinski Att0rneyRoland A. Anderson[57] ABSTRACT This alignment system utilizes intermediate photodetectorshaving central apertures and a terminal photodetector, eachphotodetector having four quadrants of active area, with their centersaligned on the axis of a laser beam, their output signals being utilizedto indicate alignment, or the degree of mis-alignment, of their centerswith respect to the axis of the laser beam. Alternatively their outputsignals are utilized to drive servo systems that automatically move thequipments on which they are mounted into alignment, Means for providingorthogonal alignment, and means to avoid air turbulence are alsoprovided.

8 Claims, 11 Drawing Figures UTlLlZATlON APPARATUSES POWER POWER 84 A 88SUPPLY SUPPLY PAIENIEUHARZY I973 SHEET 2 [IF 3 X INDICATOR Y INDICATORSUMMERS IHFD25O DIFFERENCE Y IN I OR 10 +10 X INDICATOR IOOK LN'VEYTORSMEADE A. STIRLAND JOHN A. KALINOWSKI PATENTEDHARZ'IIQYS 3 723,013

SHEET 30F 3 X INDICATOR Y INDICATOR CTRONICS-\ INVENTORS MEADE A.STIRLAND JOHN A. KALINOWSKI ALIGNMENT SYSTEM This invention relates to asystem for spatially adjusting points or objects until they are inalignment with reference to a straight line.

We made the present invention in the course of work under Contract No.AT(291)-l183 with the U.S. Atomic Energy Commission.

Thepresent invention provides a novel and improved system by whichdesired conditions of relative alignment between selected objects may bequickly, efficiently and accurately attained.

The invention provides a system that automatically determines conditionsof misalignment of objects relav tive to a sight line produced by thesystem and indicates requirements for achieving alignment.

The invention also provides a novel system for producing orthogonalalignment of a target relative to a sight line produced by the system.

Still further, the invention provides systems of the above characterwhich are unique as to their simplicity, economy of construction andprecision of operation.

Although the present alignment system has general application, it hasits greatest advantage in connection with the use of remote controldevices where human operators cannot be stationed at the points to bealigned because of inaccessibility, radiation hazard or some otherprohibitive condition. Remote indicating means capable of high precisionand appropriate correcting means operable by remote control then becomenecessary.

Thus, the invention provides an improved alignment system for indicatingthe departure of predetermined points from reference positions, providesremote control means for correcting indicated departures, provides animproved system for indicating misalignment and the effects ofcorrective efforts, and provides an improved target for such a system.

The foregoing and other advantages will become apparent and theinvention will be better understood upon perusal of the followingdescription of preferred embodiments as illustrated in the accompanyingdrawings. Various changes may be made, however, in the details andarrangement of parts and certain features may be employed withoutothers. All such modifications within the scope of the appended claimscomprise the invention.

In the drawings:

FIG. 1 is a partial block diagram and partial perspective viewillustrating schematically the system of the present invention;

FIG. 2 illustrates FIG. intermediate photodetector utilized in thesystem of the present invention;

FIG. 3 illustrates a terminal photodetector utilized in the system ofthe present invention;

FIG. 4 is a schematic diagram of an amplifier circuit that may be usedwith the photodetectors of FIGS. 2 and 3;

FIG. 5 illustrates schematically in detail one system for determiningthe orientation of the center of a photodetector with respect to a laserbeam;

FIG. 6 illustrates schematically another system for determining theorientation'of the center of a photodetector with respect to a laserbeam;

FIG. 7 is an alternative embodiment of the system of FIG. 1 adapted toproduce orthogonalalignment of a target;

FIG. 8 illustrates, in part, the system of FIG. 1 with a furthermodification;

FIG. 9 illustrates schematically the use of the invention in acollimation system; and

FIGS. 10 and 11 illustrate variations in the shapes of the areas of thephotodetectors of FIGS. 2 and 3.

FIG. 1 illustrates the system of the present invention as being utilizedto align lines L L and L, on top surfaces 22,, 22, and 22 respectively,of equipments 20 20 and 20 along straight line 24, with L of the topsurface 22 of equipment 20 offset a predetermined distance d, from line24. Note that the distances D D D etc., between equipments 20 20,, etc.may vary. In one application of the invention D was 33 feet, D, was 31feet and D was 36 feet for atom] distance of 100 feet between equipment20 and equipment 20,.

Laser 28 mounts on equipment 20 by suitable mounting means 26, with axis30 of its beam 32 directed parallel to line 24. Supply 28, powers laser28. Intermediate photodetectors 34 and 36 protrude above equipments 20and 20 Terminal photodetector 38 likewise protrudes above equipment 20,.

Referring, for the moment, to FIG. 2, intermediate photodetectors 34 and36 have four quadrants 40, each shaped as a sector of an annulus, ofphotosensitive material. For example, quadrants 40 may be siliconphotodiodes, each having terminals 42 and 44. Annulus 46 centers withinquadrants 40. An inactive mask material coats annulus 46. Aperture 48centers within annulus 46. The horizontal and vertical spacings 50between quadrants 40 are about 0.010 inch. All quadrants 40 should beequal in area.

Terminal photodetector 38, illustrated in FIG. 3, similarly has fourquadrants 52, shaped as circular sectors, of photosensitive material.Quadrants 52 similarly may be silicon photodiodes having terminals 54and 56. Terminal photodetector 38 has no center annulus of inactivemasking material and no central aperture. Again, the horizontal andvertical spacings 58 between quadrants are about 0.010 inch.

FIGS. 1, 2 and 3 also show quadrants 52 affixed to suitable substrates60, such as phenolic composition boards. This facilitates the mountingof photodetectors 34, 36 and 38 on mounting means 62, 64 and 66,respectively.

The dimensions of photodetector 34 and mounting means 62 are soselected, that when mounted on surface 22,, the center of aperture 48 ofphotodetector 34 lies on axis 30 of laser beam 32 when line L, ofsurface 22, is aligned with line 24. Moreover, when lines L L L and L,are properly aligned, surfaces 22,, 22,, 22,, and 22 are parellel ifmounting means 26, 62 64 and 66 are mounted perpendicular to saidsurfaces.

Since the divergence of laser beam 32 and distance D are known, thediameter of laser beam 32 where it impinges on photodetector 34 eitherisknown or can be calculated. The outer diameter of annulus 46 of inactivemasking material is made equal to this diameter of the laser beam. Thediameter of aperture 48 may, for example, be made equal to the diameterof the one-half power points of laser beam 32. The diameter of aperture48 should not be so small as to cause diffraction rings to infringe onphotodetector 36. In one system the diameter was 0.250 inch, the outerdiameter of annulus 46 was 0.5 inch, the outer diameter of quadrants 40was 1.12 inch, and substrate 60 had a diameter of 1.521 inch.

Laser beam 32 exits photodetector 34 with a diameter equal to thediameter of aperture 48 of photodetector 34. Again, since the divergenceof laser beam 32 and distance D are known, the diameter of laser beam 32where it impinges on photodetector 36 either is known or can becalculated. Similarly, the outer diameter of annulus 46 of inactive maskmaterial of photodetector 36 is made equal to this latter diameter oflaser beam 32. The diameter of aperture 48 of photodetector 36 maylikewise be made equal to the diameter of aperture 48 of photodetector34. This is the diameter of laser beam 32 as it exits photodetector 36.It then impinges on terminal photodetector 38, a distance D away. Again,the diameter of aperture 48 of photodetector 36 should not be so smallas to cause diffraction rings to impinge on terminal photodetector 38.

The quadrant method of monitoring laser beam 32 at photodetectors 34, 36and 38 requires that laser beam 32 meet certain requirements. Laser beammust have a very smooth continuous Gaussian distribution across the beamin all directions with a spherical distribution of the power throughoutthe wavefront. It must have a good spherical geometry and no hot spotsacross the beam front. Apertures 48 of intermediate photodetectors 34and 36 must cause the first few heavy diffraction rings that carry mostof the diffracted power to diverge less than the transmitted beam andthe pattern at the periphery of the transmitted beam must have aGaussian distribution. This is important since it is the periphery ofthe beam that falls on intermediate photodetectors 34 and 36 at thealigned position and hence it must be of uniform radial distribution.Any continuous wave or pulsed laser operating in the TEM,,,, modegenerally meets these requirements.

A laser 28 having the above characteristics is the UniversityLaboratories Model No. 240, a l-milliwatt I-Ie-Ne laser (6328 A)operating in the TEM,,,, mode with a full-angle beam divergence of 0.8milliradians and a beam width, at aperture, of 1.4 mm at the 1/e points.

With this laser and D equal to 33 feet, and with aperture 48 ofphotodetector 34 equal to 0.250 inch and aligned with the axis of laserbeam 32, aperture 48 transmits better than 80 percent of the power inlaser beam 32 and the diffraction rings imposed on the beam by aperture48 do not diverge outside the transmitted beam. With aperture 48 ofphotodetector 36 likewise equal to 0.250 inch and aligned with the axisof laser beam 32 of distance D of 31 feet from photodetector 34, thislatter aperture still transmits better than 75 percent of the initialpower onto terminal photodetector 38 located a further distance D,, awayof 36 feet.

Referring to FIG. 1 it is now appropriate to summarize briefly. Assumefirst that photodetectors 34, 36 and 38 are aligned with the axis oflaser beam 32. Laser beam 32 exits laser 28 and diverges to a diameterequal to the outer diameter of inactive mask material 46 ofphotodetector 34 as it impinges thereon. The amount of light in theoutermost periphery of beam 32 impinging on the active areas ofquadrants 40 is detected in quadrants 40, and, as will be hereinafterdescribed, is summed in an appropriate manner and indicated on a readoutsystem. After laser beam 32 exits aperture 48 of photodetector 34, itagain diverges until its diameter equals the outer diameter of inactivemask material 46 of photodetector 36 as it impinges thereon. Again, theamount of light in the outermost periphery of beam 32 impinging onquadrants 40 of photodetector 36 is detected in quadrants 40, and issummed and indicated in a readout device. After laser beam 32 exitsaperture 48 of photodetector 34, it again diverges and impinges onquadrants 52 of terminal photodetector 38, where it is detected and thensummed and indicated in a readout device.

Where quadrants 40 and 52 are silicon photodiodes, each quadrant mayrequire its own amplifier circuit 68 (such as the circuit illustrated,merely as an example, in FIG. 4) to amplify the signals produced in itup to a level suitable for operation of the electronic summing anddifference network utilized. On the other hand, amplifier 68 of FIG. 4may not be needed if signal conditions are otherwise suitable.

The circuit illustrated in FIG. 5 illustrates, by way of example, anelectronic network of summing and difference circuits connected toterminal photodetector 38. Intermediate photodetectors 34 and 36 connectto this or similar networks. Note that the individual summing anddifference circuits are conventional; hence their detailed operationwill not be described. It will be seen that summing circuit A sums thesignals from quadrants 1 and 2; B from quadrants 1 and 4; C fromquadrants 3 and 4; and D from quadrants 2 and 3. The Y output representsthe difference between the outputs of summing circuits A and C, or l 2]3 4] where these numerals represent the signals from the respectivenumbered quadrants. Similarly the X output represents the differencebetween the outputs of summing circuits B and D, or l 4] 2 3] where thenumerals represent the signals from the numbered quadrants. The outputsat X and Y connect to X and Y indicators, as illustrated, or to otherutilization apparatus as will hereinafter be described.

Obviously the converse system of FIG. 6 may be used. Here dotted circle61 represents the area of impingement of laser beam 32 on anintermediate photodetector. The signal outputs of quadrants 1 through 4are applied to a system of four difference circuits 63, the outputs ofwhich are applied to summing circuits 65 to drive X and Y indicators.

Referring again to FIG. 1, the outputs of photodetectors 34, 36 and 38connect through cables 70, 72 and 74, respectively, to electronicsassembly 76. Electronics assembly 76 contains the sum and differencenetworks needed to ascertain electronically the degree of misalignmentof photodetectors 34, 36 and 38 with respect to axis 30 of laser beam32. Cables 78, 80 and 82 connect electronic assembly 76 to utilizationapparatuses 84, 86 and 88 respectively.

Utilization apparatuses 84, 86 and 88 may, for example, each contain Xand Y indicators only. These indicate visually the X and Y directions ofmisalignment, or the alignment, of photodetectors 34, 36 and 38 withrespect to axis 30 of laser beam 32. This embodiment finds particularutility in the situation, for example, where equipments 20 20 and 20 arereadily accessidifference networks in electronic assembly 76. Suchautomatic means may, for example, comprise a servo system, in whichutilization apparatuses 84, 86 and 88 are servo drivers connected bycabling 94 to servomechanisms (not shown) contained in equipments 20,,20 and 20 for adjusting their X and Y orientations-and to power supply96 which provides power for the servo system.

We conducted a number of tests on photodetectors 34, 36 and v 38 of FIG.1 where D was 33 feet, D, was 31 feet and D was 36 feet, and quadrants40 and 52 of the photodetectors were silicon photodiodes.

Quadrants 40 of intermediate photodetectors 34 and 36 had an outerdiameter of 1.12 inch and an inner diameter of 0.50 inch, with 0.010inch spacings between quadrants. Quadrants 52 of terminal photodetector38 had an outer diameter of 1.12 inch with 0.010 inch spacings betweenquadrants.

In the first test we measured the dark current of each quadrant byconnecting it to a power supply to provide bias and to a Keithly 417picoammeter. A dark box with electrical, feedthroughs held thephotodetectors and provided light isolation. The temperature was 235C1': 1.jEach diode was reverse-biased at volts. The resulting data, shownin Table 1, indicate good matching between quadrants of individualphotodetectors, and very similar matching between photodetectors.

- TABLE 1 Dark current of photodetectors. Data taken at 23.5 C i 1 C.Diodes reverse biased at 10 V.

Intermediate Photodetector 34 Dark Current After assemblying the systemof FIG. 1 we determined theindividual response of photodetectors 34, 36and 38 to offsets to each of 0.01 inch in each of the four cardinaldirections, :X and i- Y. This was done at each photodetector withreadingsat the X-Indicator of the circuit of FIG. 5 for thatphotodetector for offsets in the X direction and readings at theY-Indicator for offsets in the Y direction. The data, shown in Table 2,indicate a very good balance of photodetector output in response toextent of offset of the photodetector from laser beam 32. The value ofthe response of terminal photodetector 38 is noticeably higher than theresponses of intermediate photodetectors 34 and 36. This results becausequadrants 52 of photodetector 38 have much more area than quadrants40 ofphotodetectors 34 and 36. Consequently, quadrants 52 intercept much morelight.

TABLE 2 Response of photodetectors 34, 36 and 38 to 0.010 inch offsets.

Intermediate Photodetector 34 Direction Reading +1: 3.25 V x +3.25 V +y3.25 V y +3.25 V

Intermediate Photodetector 36 +1: 4 V .t +4 V +y 4 V y +4 V TerminalPhotodetector 38 +1: 8.5 V -x +8.5 V +y 8.5 V y +8.5 V

Next, we entered offsets to intermediate photodetector 34 and measuredthe responses of the three photodetectors 34, 36 and 38. Then, weentered offsets to intermediate photodetector 36 and measured theresponses of photodetectors 36 and 38. Last, we entered offsets toterminal photodetector 38 and measured its responses. The data arepresented in Table 3.

Note than when photodetector 34 was offset in a positive X- direction,the response for photodetector 34 was a negative voltage, the responsefor photodetector 36 was a positive voltage and the response forterminal photodetector 38 began as a negative voltage for the first fewthousandths of an inch of offset and then changed to a positive voltage.Consider what happens. When intermediate photodetector 34 is offset inpositive X-direction, it intercepts more of the negative side of laserbeam 32 which impinges on quadrants 3 and 4, producing negativevoltages. Power is extracted from that portion of beam 32. As thetransmitted beam proceeds to intermediate photodetector 36, the beam isweak on that side and the response of photodetector 36 indicates thedecrease of power in that portion of the beam by producing positivevoltages. However, when the beam impinges on terminal photodetector 38its geometry has been changed by beam divergence, diffraction and otherfactors to where the majority of the power is again in the negative sideof the beam for the first few thousandths of an inch of offset. Togeneralize, when an offset is positive (negative), the response ofphotodetector 34 is negative (positive) and the responses ofphotodetectors 36 and 38 are positive (negative).

TABLE 3 Response of photodetectors 34, 36 and 38 to offsets along majoraxes Intermediate Photodetector 34 Offset in -x direction, x-readingphotodetectors 34, 36 and 38 Offset Photode Photodetector 36Photodetector 38 tector 34 0.025 in. +0.50 V l.00 V +1.00 V

0.050 in. 0.075 in. 0.100 in. 0.125 in. 0.150 in.

Offset in +x direction, x-reading photodetectors 34, 36

and 38 Offset 0.025 in 0.050 in. 0.750 in. 0.100 in. 0.125 in. 0.150 in.0.175 in.

Offset in +y direction, y-reading photodetectors 34, 36

and 3 8 Offset 0.025 in. 0.050 in. 0.075 in. 0.100 in. 0.125 in. 0.150in, 0.175 in.

Offset in y direction, y-reading photodetectors 34, 36

and 38 0.025 in. 0.050 in. 0.075 in.

0.100 in. 0.125 in.

Offset in x direction, x-reading photodetectors 36 and Offset 0.025 in.0.050 in. 0.075 in. 0.100 in. 0.125 in. 0.150 in.

Offset in +x direction, x-reading photodetectors 36 and 0.025 in. 0.050in. 0.075 in. 0.100 in. 0.125 in. 0.150 in.

Offset in +y direction, y-reading photodetectors 36 and Offset 0.025 in.0.050 in. 0.075 in. 0.100 in. 0.125 in. 0.150 in.

Photode tector 34 -.50 V 1.00 V 1.50 V 3.00 V 4.75 V 7.50 V 11 (offscale) Photode tector 34 Photodetector 36 +1.25 V +2.50 V +3.75 V +4.25V +4.50 V +4.50 V +4.00 V

Photodetector 36 +1.25 V +2.50 V +3.75 V +4.50 V +4.75 V +4.50 V +4.50 V

hotodetector 36 --1.00 V 2.00 V 3.00 V 3.50 V 3.50 V 3.75 V

Photodetector 38 +2.75 V +5.50 V +3.75 V +1.50 V +0.50 V

Photodetector 38 +3.75 V +5.00 V +3.50 V +1.25 V +0.25 V

Photodetector 38 Intermediate Photodetector 36 Photodetector 36 +1.00 V

+11 (011 scale) Photodetector 36 0.75 V 1.75 V 3.50 V 5.50 V

Photodetector 36 0.75 V

Photodetector 38 3.50 V

Photodetectors 38 +4.00 V

+11 V (off scale) +10.00 V

Photodetector 38 +4.00 V

+1000 V +l0.00 V

Offset Photodetector 36 Photodetcctor 38 0.025 in. +0.75 V 4.50 V 0.050in. +2.00 V 7.50 V 0.075 in. +3.50 V 10.00 V 0.100 in. +5.50 V 11 V(offscale) 0.125 in. +8.00 V 10.00 V 0.150 in. +1000 V 9.00 V

Terminal PI-lotodetector 38 Offset x direction, x-reading photodetector38 Offset Photodctector 38 0.025 in. +2.00 V 0.050 in. +4.25 V 0.075 in.+6.00 V 0.100 in. +9.00 V 0.125 in. +11 V (off scale) Offset in +xdirection, x-reading photodetector 38 Offset Photodetector 38 0.025 in2.50 V 0.050 in. 4.50 V 0.075 in. 7.00 V 0.100 in. 9.50 V

Offset in +y direction, y-reading photodetector 38 Offset Photodetector38 0.025 in. 3.00 V

0.050 in. 5.75 V

0.075 in. 7.50 V

0.100 in. 10.00 V

Offset in y direction, y-reading photodetector 38 Offset Photodetector38 0.025 in. +2.25 V

0.050 in. +4.50 V

0.075 in. +6.50 V

0.100 in. +8.00 V

0.125 in. +10.00 V

Next we verified the response of photodetectors 34, 36 and 38 with thequadrants of each photodetector rotated through angles of 30, 45 and 60in a counterclockwise direction. Note that quadrants 40 and 52 arenumbered in Cartesian fashion. After each photodetector was rotated, itwas offset 0.141 inch for the 45 angle of rotation and 0.1 12 inch forthe 30 and 60 angles of rotation. The data for the 45 readings arepresented in TAble 4 and data for the 30 and 60 readings in Table 5.

TABLE 4 Photodetector response at 45. lntennediate Read- Photode-Photode- Photode- Photodetector ing tector 34 tector 36 tector 38 34 45Quad 1 X +7.5 V 3.50 V l.50 V Y +7.5 V 3.S0 V -1.50 V 45 Quad 2 X 6.0 V+4.00 V +1.75 V Y +6.0 V -4.00 V 2.00 V 45 Quad 3 X 7.0 V +4.25 V 1.50 VY 7.0 V +4.25 V 1.50 V 45 Quad 4 X +8.0 V 3.50 V 1.50 V Y 8.0 V +3.50 V+1.25 V Intermediate Read- Photodetector Photodetector ggotodetector ing38 Quad 1 X +7.00 V 7.5 V

Y 6.75 V 7.5 V Quad 2 X 7.50 V +7.0 V Y +7.50 V 7.5 V Quad 3 X 8.00 V+7.5 V Y 8.00 V +7.5 V Quad 4 X +7.00 V -6.5 V Y 7.00 V +6.5 V TerminalRead- Photodetector Photodetector ing 38 38 was masked in the signal. Anull indication on the X- and Ye lower case for terminal photodetector38 indicated that laser beam 32 was indeed centered on terminalphotodetector 38. With beam 32 blocked the 0.5 volt negative Y offsetwas obtained.

In the system of FIG. 1, electronics assembly 76 contains the logic,steering, and other control circuits (not shown) required to distinguishbetween normal operation when laser beam 32 actually impinges on allfour quadrants 52 of terminal photodetector 38 and the conditionexisting when laser beam 32 is obstructed, producing the negative Youtput. The output of such circuits, when this condition exists,triggers an alarm, or, as illustrated in FIG. 1, supplies current alongline 98 to light lamp 100 which is located on the operators console.These circuits also inhibit the transmission of signals to servo drivers84, 86 and 88.

FIG. 7 illustrates, in part, a modification of the system of FIG. 1wherein beam splitter 102 divides beam 32 into two beams 32, and 32,having orthogonal axes 30 and 30 It will be understood that axis 30, isparallel to original axis 30, but is not coincident therewith.Intermediate photodetector 104 and terminal photodetector 106 providesignals indicating the degree of misalignment, of alignment, or theequipment (not shown) on which they are mounted with respect toorthogonal axis 30 It will be obvious that beam splitter 102 may be sodisposed that axes 30 and 30 are not orthogonal. Moreover, beam splitter102 may be located between intermediate photodetectors 34 and 36 orbetween intermediate photodetector 36 and terminal photodetector 38.

FIGS. 8 illustrates, in part, still a further modification to the systemof FIG. 1 used to eliminate air turbulence in the line of sight betweenlaser 28, intermediate photodetectors 34 and 36, and terminalphotodetector 38. The modification utilizes pipe 108 to enclose the lineof sight.

FIG. 9 illustrates schematically use of the invention in a collimatingsystem. Here intermediate photodetector 110 mounts on laser 1 12. Theapertures of photodetector 110 and laser 112 may coincide, as at 114, ifpreferred. Laser beam 116 having an axis 118 exits aperture 114 andimpinges on mirror 120. If the planar incident surface of mirror 120upon which laser beam 116 impinges is perpendicular to axis 118, laserbeam 116 will be reflected back to impinge on equal areas of the fourquadrant diodes of intermediate photodetector 110. In such event theoutputs to electronics package 122 are equal and there will be noindications on the X and Y indicators. If the axis of the reflected beamdoes not coincide with axis 118, mirror 120 is not perpendicular to axis118 and the X and Y indicators will so indicate. Adjustment of mirror120 to zero the X and Y indicators will correct this condition.Obviously, servo systems can be utilized to maintain precise collimationautomatically.

FIGS. 10 and 11 illustrate variations in the shapes of the areas ofphotosensitive materials in the photodetectors. Thus, in FIG. 10intermediate photodetectors 34 or 36' have four photodiodes 40' shapedas illustrated and spaced equally at 90 intervals around aperture 48.Likewise, in FIG. 11 photodiodes 52' are spaced at 90 intervals aroundthe center of terminal photodetector 38'. For simplicity, eachphotodetector should have at least four equal areas of photosensitivematerial spaced at intervals around its center. Obviously, multiples offour areas may be used.

While FIG. 1 illustrates axis 30 of laser beam 32 as being directedparallel to line 24, it will be understood that axis 30 may be directedin a predetermined direction with respect to line 24. Equipments 20,, 2020;, and 20., may then be aligned in the predetermined direction withrespect to line 24.

This invention may be embodied in other ways without departing from thespirit or essential character thereof. The embodiments of the inventiondescribed herein are therefore illustrative and not restrictive, thescope of the invention being indicated by the appended claims and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

The invention claimed is:

l. A system for aligning a plurality of equipments in a predetermineddirection with respect to a reference line of sight comprising:

a laser disposed with the axis of the laser beam directed in thepredetermined direction; a terminal photodetector having four equalareas of light-sensitive material spaced at 90 intervals around itscenter mounted on a first equipment, which is the most distant from thelaser of the plurality of equipments to be aligned, so that the areasface the laser beam and when the terminal photodetector is centered onthe axis of the laser beam the first equipment is aligned in thepredetermined direction; at least one intermediate photodetector mountedon an intermediate equipment located at a point between the laser andthe first equipment, the photodetector having a substrate with anaperture, four equal areas of light-sensitive material on the substratespaced at 90 intervals around the aperture and an annulus of inactivematerial forming a central mask between the aperture and thelight-sensitive areas,

the intermediate photodetector being so mounted that the areas and maskface the laser beam and when the intermediate photodetector is centeredon the axis of the laser beam the intermediate equipment is aligned inthe predetermined direction;

first electrical means connected to the areas of the terminalphotodetector for producing signals indicative of the relative positionof the terminal photodetector with respect to the axis of the laser beamwhen the laser beam impinges on the areas and for producing adistinguishable signal when the laser beam does not impinge upon theareas;

second electrical means connected to the areas of the intermediatephotodetector for producing, when the laser beam impinges on its areas,signals indicative of its relative position with respect to the axis ofthe laser beam; and

means for moving the terminal and intermediate equipments into alignmentwith the predetermined direction.

2. The system of claim 1 wherein the first electrical means produce nullindications in the X and Y directions established by the four equalareas of lightsensitive material when the laser beam impinges on thephotodetector and the photodetector is in alignment therewith andproduce a predetermined signal in one of either the X or Y directionwhen the laser does not impinge upon the areas.

3. The system of claim 2 wherein the predetermined signal produced whenthe laser beam does not impinge upon the areas is obtained by adjustmentof the dark current of one of the areas of light-sensitive material.

4. The system of claim 2 wherein the null indication and predeterminedsignal are obtained by the adjustment of the dark current of the fourlight-sensitive areas to an unbalanced condition whereby when the laserbeam does not impinge upon the photodetector a signal is obtained in onedirection only.

5. In a system for aligning equipment in a predetermined direction withreference to a line of sight established by a laser beam which includesa photodetector having four equal areas of light-sensitive materialaround its center mounted on equipment to be aligned, the improvementcomprising electrical means connected to said areas of a photodetectorfor producdirections established by the four equal areas oflightsensitive material when the laser beam impinges on thephotodetector and the photodetector is in alignment therewith andproduce a predetermined signal in one of either the X or Y directionwhen the laser beam does not impinge upon the areas.

7. The improvement of claim 6 wherein the predetermined signal producedwhen the laser beam does not impinge upon the areas is obtained byadjustment of the dark current of one of the areas of light-sensitivematerial.

8. The system of claim 6 wherein the null indication and predeterminedsignal are obtained by the adjustment of the dark current of the fourlight-sensitive areas to an unbalanced condition whereby when the laserbeam does not impinge upon the photodetector a signal is obtained in onedirection only.

1. A system for aligning a plurality of equipments in a predetermineddirection with respect to a reference line of sight comprising: a laserdisposed with the axis of the laser beam directed in the predetermineddirection; a terminal photodetector having four equal areas oflightsensitive material spaced at 90* intervals around its centermounted on a first equipment, which is the most distant from the laserof the plurality of equipments to be aligned, so that the areas face thelaser beam and when the terminal photodetector is centered on the axisof the laser beam the first equipment is aligned in the predetermineddirection; at least one intermediate photodetector mounted on anintermediate equipment located at a point between the laser and thefirst equipment, the photodetector having a substrate with an aperture,four equal areas of light-sensitive material on the substrate spaced at90* intervals around the aperture and an annulus of inactive materialforming a central mask between the aperture and the light-sensitiveareas, the intermediate photodetector being so mounteD that the areasand mask face the laser beam and when the intermediate photodetector iscentered on the axis of the laser beam the intermediate equipment isaligned in the predetermined direction; first electrical means connectedto the areas of the terminal photodetector for producing signalsindicative of the relative position of the terminal photodetector withrespect to the axis of the laser beam when the laser beam impinges onthe areas and for producing a distinguishable signal when the laser beamdoes not impinge upon the areas; second electrical means connected tothe areas of the intermediate photodetector for producing, when thelaser beam impinges on its areas, signals indicative of its relativeposition with respect to the axis of the laser beam; and means formoving the terminal and intermediate equipments into alignment with thepredetermined direction.
 2. The system of claim 1 wherein the firstelectrical means produce null indications in the X and Y directionsestablished by the four equal areas of light-sensitive material when thelaser beam impinges on the photodetector and the photodetector is inalignment therewith and produce a predetermined signal in one of eitherthe X or Y direction when the laser does not impinge upon the areas. 3.The system of claim 2 wherein the predetermined signal produced when thelaser beam does not impinge upon the areas is obtained by adjustment ofthe dark current of one of the areas of light-sensitive material.
 4. Thesystem of claim 2 wherein the null indication and predetermined signalare obtained by the adjustment of the dark current of the fourlight-sensitive areas to an unbalanced condition whereby when the laserbeam does not impinge upon the photodetector a signal is obtained in onedirection only.
 5. In a system for aligning equipment in a predetermineddirection with reference to a line of sight established by a laser beamwhich includes a photodetector having four equal areas oflight-sensitive material around its center mounted on equipment to bealigned, the improvement comprising electrical means connected to saidareas of a photodetector for producing signals indicative of therelative position of the photodetector with respect to the axis of thelaser beam when the laser beam impinges on the areas and for producing adistinguishable signal when the laser beam does not impinge upon theareas.
 6. The improvement of claim 5 wherein the electrical meansproduce null indications in the X and Y directions established by thefour equal areas of light-sensitive material when the laser beamimpinges on the photodetector and the photodetector is in alignmenttherewith and produce a predetermined signal in one of either the X or Ydirection when the laser beam does not impinge upon the areas.
 7. Theimprovement of claim 6 wherein the predetermined signal produced whenthe laser beam does not impinge upon the areas is obtained by adjustmentof the dark current of one of the areas of light-sensitive material. 8.The system of claim 6 wherein the null indication and predeterminedsignal are obtained by the adjustment of the dark current of the fourlight-sensitive areas to an unbalanced condition whereby when the laserbeam does not impinge upon the photodetector a signal is obtained in onedirection only.